Do Energy Efficient Skylights Lower Heating Bills?

A rooflight is judged on its whole-unit U-value, which is not the same figure as the glass alone. Heat escapes through the centre of the pane, through the frame around it, and through the edge of the sealed unit where the spacer bar sits between the two or three panes of glass. Our guide to reading a rooflight U-value goes into how whole-unit and centre-pane figures are worked out and why a brochure quoting the better of the two can be misleading.

Older single-glazed rooflights, and the plastic domes fitted to many flat-roofed extensions before double glazing became standard, sit at the weak end of that scale: a single pane of glass with no coating and an uninsulated frame loses heat quickly on a cold night, whatever room sits underneath it. A modern sealed unit closes most of that gap before the frame or the seal are even considered.

Between the panes of a current rooflight sits a low-E coating, a microscopically thin metallic layer that lets daylight through but reflects room heat back inward, and usually a fill of argon or a similar inert gas in place of ordinary air, which conducts heat more slowly across the cavity. Our page on low-E coatings and gas-filled glass sets out how the two work together without dimming the view through the glass.

Moving from two panes to three adds a second cavity and a second gap for the coating and gas fill to do their work in, which is why a well-specified triple-glazed rooflight typically posts a lower whole-unit U-value than an equivalent double-glazed one. Our comparison of two panes or three weighs that gain against the extra weight and depth a third pane brings to the frame.

The spacer bar that holds the two or three panes apart at their edge used to be a strip of aluminium, an excellent conductor and a poor choice for anything meant to keep heat in. A cold spacer pulls the temperature of the glass down at its edge before the middle of the pane, which is usually where the first misting or condensation appears on a winter morning. Our guide to warm edge spacers explains how a low-conductivity spacer keeps that edge closer to room temperature and cuts the heat lost there.

The frame around the glass carries the same problem at a larger scale. A frame without a thermal break, an insulating strip separating its inner and outer faces, conducts heat straight through from the warm room to the cold roof outside. Our guide to thermally broken frames and cold bridging covers how that break is built in and why it also stops condensation forming on the inner frame face, which a cold, unbroken frame does reliably on a cold night.

A bigger rooflight lets in more daylight and more sky, but it also has more area for heat to cross through in both directions: more solar gain on a bright day, and more heat lost through the glass on a cold, dark one. Getting that balance right is less about a rule of thumb and more about the room underneath it.

• How the room is used, and whether it needs steady warmth through the evening or mainly daylight during the day.
• Which way the roof slope faces, since a south-facing rooflight picks up more winter sun than a north-facing one on the other side of the same house.
• How well insulated the rest of the roof and walls already are, since a single weak point matters more in an otherwise tight house.
• Whether solar control glass is worth adding alongside the low-E coating, covered on our page on solar control glass, to manage gain without giving up warmth in winter.

Our guide to balancing daylight and heat loss when sizing works through that trade-off room by room, and our answer on whether north-facing skylights lose more heat looks specifically at orientation, since night-time heat loss through the glass carries on regardless of which way the roof faces once the sun has gone down.

Part L of the Building Regulations sets a maximum whole-unit U-value that a new or replacement rooflight has to meet, which is why a current sealed unit installed to current standards already sits well below what an older single-glazed rooflight ever managed. Our guide to what Part L asks of a rooflight sets out that figure and how it is checked at installation.

On a new build or an extension, the rooflight's U-value also feeds into the SAP calculation used to demonstrate the whole house meets its energy target, alongside the walls, the roof and every other opening. Our answer on whether rooflights affect SAP calculations explains how a single rooflight's spec can matter to an assessment covering the entire house, not just the room it sits above.

The largest single step in heat loss through a rooflight is usually the one between an old single-glazed unit, or a tired plastic dome from before double glazing was standard, and a current double or triple-glazed unit built to today's Part L figure. Our guide to replacing single-glazed rooflights with modern glazing sets out what changes when that swap is made, glass, frame and edge together rather than the glass on its own.

We work through the glazing, frame and sizing decisions at survey and put the sightline through to the sky on the same drawing, so the reasoning behind the spec is visible alongside the view it gives. See our energy efficient rooflights page for how that spec is put together, and our page on the sightline sketch for how the drawing itself works. We install to current Building Regulations and handle the Building Control notification where the work is notifiable, with a 10-year workmanship guarantee on the finished job.

Does a bigger rooflight always lose more heat than a small one?

Area alone is not the whole story. A large rooflight built to a low whole-unit U-value can lose less heat overall than a smaller one glazed to an older, weaker spec, which is why the glazing generation matters as much as the size.

Is triple glazing always worth it for heating bills alone?

It brings a lower U-value than double glazing, but it also adds weight, depth and typically a higher spec cost, so it is weighed against the room's own needs rather than fitted as a default. Our comparison of two panes or three sets out that trade-off.

Can an existing double-glazed rooflight be upgraded without replacing the whole unit?

The sealed unit itself, glass, coating, gas fill and spacer together, is generally replaced as one piece rather than altered in place, since the coating and gas fill are built in at manufacture. The frame around it can sometimes be reused if it is already thermally broken and sound.

Does the room below a rooflight need extra insulation once the glazing is upgraded?

The reveal and lining around the frame are built up with insulation as part of the installation, so the glazing and the surrounding fabric are addressed together rather than one after the other.